Conventionally, means for performing wavelength selection (line selection) for laser light with use of a dispersing prism, and narrowing spectral line width thereof is known, and, for example, document, Canadian Journal of Physics Vol.63.1985 (pp.214–219) shows the one. FIG. 10 shows a plan view of a molecular fluorine laser device in which spectral line width is narrowed with use of line select means disclosed in the aforementioned document, and the prior art will be explained hereinafter based on FIG. 10.
In FIG. 10, a molecular fluorine laser device 11 includes a laser chamber 12 in which a laser gas that is a laser medium is sealed. The laser gas contains fluorine (F2) and a buffer gas with predetermined composition. As the buffer gas, helium (He), neon (Ne), or a mixture gas of both of them is generally used. A pair of discharge electrodes 14 and 15 are placed to oppose each other perpendicularly to the paper surface of FIG. 10 inside the laser chamber 12. The discharge electrodes 14 and 15 are connected to a high voltage power supply (not shown), and high voltage is applied to the discharge electrodes 14 and 15. Consequently, a pulse discharge occurs between the discharge electrodes 14 and 15, which excites the laser gas to oscillate molecular fluorine laser light 21 (hereinafter, called the laser light 21) in a pulse form.
A front window 17 and a rear window 19, which transmit the laser light 21, are provided at a front and rear parts of the laser chamber 12. A front slit 26 and a rear slit 27, which have openings of predetermined width, are placed in front (the right side in FIG. 10) of and behind the laser chamber 12. A front mirror 16, which partially transmits the laser light 21, is placed in front of the front slit 26. For example, two dispersing prisms 50 and 50 are placed behind the rear slit 27, and a rear mirror 18, which totally reflects the laser light 21, is placed behind the dispersing prisms 50 and 50.
In FIG. 10, 44 denotes a fixing plate for fixing the dispersing prisms 50 and 50. Rods 45 with tip ends being threaded are protruded from a base (not shown) around the dispersing prisms 50 and 50. Each of the dispersing prisms 50 is pressed from above by the fixing plate 44 and nuts 46 to be fixed. Namely, if an adhesive or the like is used to fix the dispersing prism 50 in the laser device which emits the laser light 21 with an ultraviolet ray wavelength, such as the molecular fluorine laser device 11, and an excimer laser device, the adhesive reacts with the laser light 21 and impurities occur. In order to prevent optical components such as the dispersing prism 50 from being contaminated by the impurities, it is necessary to fix the dispersing prism 50 by using only a force by pressing or the like in these laser devices.
The laser light 21 oscillated in the laser chamber 12 passes through the windows 17 and 19, and the dispersing prisms 50 and 50, and is amplified while it is reflected and reciprocated between the rear mirror 18 and the front mirror 16, and part of it is transmitted through the front mirror 16 and taken out. At this time, strong line light (center wavelength 157.63 nm) with a long wavelength, and weak line light (center wavelength 157.52 nm) with a short wavelength are mixed in the laser light 21. Since strong line light and weak line light differ in wavelength, a difference occurs to a refraction angle of an optical path transmitted through the dispersing prisms 50 and 50. Consequently, the optical paths of the strong line light and weak line light are gradually deviated while they pass through two of the dispersing prisms 50 and 50.
As a result, the strong line light passing through the dispersing prisms 50 and 50 passes through openings of the slits 26 and 27 to be emitted from the front mirror 16. On the other hand, the weak line light has its optical path deviated while passing through two of the dispersing prisms 50 and 50 and is shielded by the front slit 26 and the rear slit 27, and is not oscillated. In the molecular fluorine laser device 11, only the strong line light is oscillated in this manner, whereby the spectral line width of the laser light 21 is narrowed, and resolution when the molecular fluorine laser device 11 is used for exposure is improved.
However, the above-described prior art has the problems as described as follows. Namely, in the prior art, as shown in FIG. 10, the individual dispersing prisms 50 and 50 are held by the fixing plates 44 and 44. Consequently, a large space is required around the dispersing prisms 50 and 50, and a space between the dispersing prisms 50 and 50 is made larger, thus making the distance the laser light 21, which is emitted from the laser chamber 12 rearward, travels until it reaches the rear mirror 18 becomes long. The laser light 21 emitted from the laser chamber 12 does not receive energy by pulse discharge, and therefore it is never amplified. In addition, resonator length that is the distance between the front mirror 16 and the rear mirror 18 becomes longer, and therefore there arises the problem that output power of the laser light 21 is reduced by the diffraction loss.
Further, when the laser light 21 emitted from the molecular fluorine laser device 11 is used for exposure, the laser light 21 has to be oscillated at high repeated frequency for a long period of time. For this purpose, it is necessary to reduce a width dimension L1 in a vertical direction in FIG. 10 of the discharge electrodes 14 and 15 of the molecular fluorine laser device 11, and increase a space between the opposing discharge electrodes 14 and 15 (in the perpendicular direction to the paper surface of FIG. 10). As a result, a beam sectional form of the laser light 21 is oblong, with a width dimension L2 in a horizontal direction with respect to the paper surface in FIG. 10 being 3 mm, and a height dimension (not shown) in the perpendicular direction to the paper surface in FIG. 10 being 20 mm. Namely, the dispersing prisms 50 and 50 on which the laser light 21 is incident also need to have the height in the perpendicular direction to the paper surface in FIG. 10 made 20 mm or higher. Accordingly, in order to hold the dispersing prisms 50 and 50 with stability, it is necessary to use the dispersing prisms 50 and 50 with large bottom areas and make the installation areas in contact with a base (not shown) large. As a result, the resonator length becomes longer, and the output power of the laser light 21 is reduced.
On the other hand, if the dispersing prisms 50 and 50 with the small bottom areas are used to prevent reduction of the output power of the laser light 21, stability of the dispersing prisms 50 and 50 is decreased, and the incidence planes are sometimes inclined with respect to an optical axis of the laser light 21. As a result, a wavefront is disturbed to cause wave aberration, and line select is not favorably performed, or reduction in output power of the laser light 21 is sometimes caused.
According to FIG. 10, the entire dispersing prisms 50 and 50 are pressed by the fixing plates 44 and 44. Consequently, the force is also exerted on the parts through which the laser light 21 is transmitted, and therefore a distortion sometimes occur to insides of the dispersion prisms 50 and 50. As a result, birefringence is caused to change the optical path of the laser light 21, or disturb the wavefront, whereby line selection is not sometimes performed favorably. In addition, the output power of the laser light 21 is sometimes reduced. If the dispersing prisms 50 and 50 are pressed with a small force to prevent this, it sometimes happens that the dispersing prisms 50 and 50 are deviated from the optical axis by vibrations and the like, thus reducing the output power of the laser light 21 or causing variations in the output power.